This is a discussion on USB to 3.5mm audio plug adapter (I want USB headset to plug into headphone jack) within the Gadget Support forums, part of the Tech Support Forum category. Hello!
I have been searching for an adapter that would allow me to plug in a USB headset into it

I don't believe that this is possible. 3.5mm jacks carry an audio signal only. The USB controller in your headset needs to talk to the system through the computer's USB controller in order to work, because it has its own audio controller chip.

What you could do, if you are technically inclined, is cut the cord that leads out of the USB adapter of your headset and solder the leads onto a 1/8" plug (or two, assuming the headset has a microphone). This method might take a little trial and error if there's a mic and they aren't in separate sheaths.

I'm not looking to change the USB port permanently. I don't mind if the headset (the USB headset includes both a set of earbuds as well as a microphone) only supports the microphone option when it's converted.

I saw a product that was similar to my own headset that had a converter like the one I want. But of course I didn't want to purchase that converter with the headset, only by itself.

Please let me know if there is anything non-permanent I can possibly do. I don't mind if I lose some functionality; like I mentioned earlier, I'm fine with using the headset as only headphones when converted to a 3.5mm jack. Also, if there's a jack out there that can convert the USB port to two plugs - one headphone and one mic-in, that would work perfectly.

I'm not looking to change the USB port permanently. I don't mind if the headset (the USB headset includes both a set of earbuds as well as a microphone) only supports the microphone option when it's converted.

I saw a product that was similar to my own headset that had a converter like the one I want. But of course I didn't want to purchase that converter with the headset, only by itself.

Please let me know if there is anything non-permanent I can possibly do. I don't mind if I lose some functionality; like I mentioned earlier, I'm fine with using the headset as only headphones when converted to a 3.5mm jack. Also, if there's a jack out there that can convert the USB port to two plugs - one headphone and one mic-in, that would work perfectly.

I've seen headsets that have two 1/8" connectors that plug into a usb adaptor for this purpose, but I haven't seen anything that allows an analog passthrough like you're talking about. Not that it can't be done, but I don't believe it would be possible to retrofit into yours without an even more invasive hack than taking the usb stick off and putting on the plugs would be.